There are several types of joints in the human body. These can be categorized into weight bearing and non-weight bearing joints. The hip, knee, ankle and intervertebral disc in the spine are considered load-bearing joints, while the finger and toe are considered non-weight bearing joints. The hip, knee, and ankle are further categorized as synovial joints, while the intervertebral disc is a cartilaginous joint. These joints, especially the weight bearing joints, can undergo degenerative changes due to disease, age, trauma, repetitive loading and/or genetics.
The individual whose joints experience such degeneration may incur significant discomfort, pain and even disability. Initially, the only option for the patient with degenerative changes to these joints was to undergo arthrodesis, or fusion, of the effected joint. Although this can effectively relieve pain and lead to an increase in the quality of life, fusion can significantly alter the normal biomechanical function of the effected joints. Treatment options have since advanced to include motion preserving implants, known as arthroplasty devices. These joint replacement devices usually comprise a pair of endplates with some type of intermediate components or articulating bearing surfaces to facilitate motion between the adjacent vertebral bodies.
One challenge for arthroplasty devices, whether for the hip, knee, ankle or spine, is the selection of the proper materials for the various components thereof. Biocompatibility—the suitability of a material for exposure to the body or bodily fluids—and biodurability—the ability of a material to maintain its physical and chemical integrity after implantation into living tissue—are essential for permanent medical implants. Materials chosen should avoid cytotoxicity, systemic toxicity, irritation, macroscopic or allergic reactions, muscle degeneration, or other adverse response. The biocompatibility and biodurability requirements significantly limit the selection of materials available for weight bearing devices.
The implant components must also exhibit sufficient strength and excellent fatigue performance to avoid mechanical failure over a long life under physiological loadings and kinematics. Properties such as yield strength, break strength, flexural strength, shear strength, and compressive strength of the implant components can significantly impact the success of the implant in weight bearing joint arthroplasty. Hard and stiff materials, such as ceramics or metals, have favorable strength characteristics. However, such materials have substantially higher flexural modules than that of cortical bone. This can cause a phenomenon known as “stress shielding,” which may cause bone loss and the loosening and eventual failure of the implants. Certain polymer materials, having a flexural modulus similar to cortical bone, are thought to minimize stress shielding and the associated adverse effects. However, many polymers do not have sufficient yield strength to be used in weight bearing joints.
As exemplified in the devices described above, known hip, knee and ankle arthroplasty devices, and the majority of disc arthroplasty devices, incorporate articulation in their design. The articulation can be conforming, such as the ball and socket arrangement of the hip joint, or non-conforming, which permits sliding motion such as in known knee arthroplasty designs. In both conforming and non-conforming designs, the motion of the articulation surfaces against each other generates wear particulate. The primary wear that occurs in a hip prosthesis is between the femoral head and the acetabular cup. In a knee prosthesis, wear occurs primarily between the distal femoral condyles and the articulation surface of the tibial tray. The generation of wear particulate is important not only from a device lifetime perspective, but also from a biological perspective. In some cases, the biological response will dictate the lifetime of the device. This is because the generation of wear particulate in sufficient amount and size may lead to an adverse cellular response, manifested by macrophage activation, giant cell formation and a cascade of cytokine release ultimately leading to an imbalance in osteoclast and osteoblast activity. This may lead to inflammation of the tissue around the reconstructed joint, osteolysis and failure of the implant.
The use of ultra-high molecular weight polyethylene (UHMWPE) against metal in total joint replacements has a long clinical history dating back decades. UHMWPE was proposed as a counterface to stainless steel due to its greater biocompatibility and increased wear resistance over PTFE when evaluated on pin-on-plate wear testing simulators. UHMWPE also possesses superior mechanical toughness and wear resistance over most other polymers. UHMWPE on metal hip joints have succeeded clinically, with high rates of survivorship beyond 25 years in some cases. However, UHMWPE is also known to have certain drawbacks and limitations. These include the need for small diameter head sizes to reduce the frictional torque due to less than optimal lubrication, oxidation of the UHMWPE resulting from ionizing sterilization, and wear caused by third body debris such a bone particulate.
One disadvantage of UHMWPE is the accumulation of wear debris eliciting an adverse cellular response leading to inflammation and osteolysis of the surrounding bone. The literature suggests a threshold wear rate of 80 mm3/year, above which particle induced osteolysis may lead to failure. The clinical wear rate of UHMWPE hip implants can potentially exceed this value. It has been suggested that the UHMWPE wear volume can be controlled below the indicated threshold for osteolysis by limiting the diameter of the femoral head. However, a smaller head decreases the range of motion of the joint and elevates the risk of the neck of the femoral stem impinging upon the cup causing dislocation of the femoral head.
The performance of UHMWPE on metal joint implants may also be adversely impacted by third-body wear particulate. For example, cements such as Polymethylmethacrylate (PMMA) are commonly used to secure the metal femoral stem of a hip prostheses into the femoral canal or the metal backing of the tibial tray to the tibial canal. PMMA particles can become entrapped between the head and UHMWPE acetabular cup. Such third-body wear particulate can also comprise bone or metal particles. This may lead to accelerated wear of the UHMWPE in such bearing couplings, either as a result of the abrasive effect of the particulate on the UHMWPE surface and/or by roughening the surface of the metal head bearing surface.
Ceramic on ceramic bearings have been found to have the lowest in vivo and in vitro wear rates to date of any bearing combination. Ceramic bearings do not share the same biological concerns from generated particulate debris as metal bearings, as they are considered to be relatively biologically inert. However, ceramics are prone to material failure when subjected to high mechanical stress, either in tensile or impact loading, which may limit their long term potential total weight bearing joint arthroplasty.
Other weight bearing joint replacement devices have been proposed that utilize compliant bearing surfaces provided as coatings of metal structural components. For example, one known attempt involved the use of a compliant material as a surface covering of a metal femoral ball articulating against the native cartilage of the acetabulum. Materials for use have included silicon rubbers, polyurethanes and olefin based synthetic rubbers. These devices have been shown to operate with very low friction because of the fluid-film lubrication that they exhibit, and therefore should produce lower wear than current prosthetic materials as the two surfaces of the joint are completely separated by a film of synovial fluid. They have been shown to possess a balance of physical strength, flexibility, dynamic flexural endurance, inherent chemical stability and physiological compatibility. The use of elastomers such as polyurethane as an articulating weight bearing material have not shown any benefit in terms of wear resistance over the more traditional bearing couples, and this may lead to questions regarding their biodurability and subsequent biocompatibility.
Thus, there is a need for weight bearing total joint arthroplasty devices having excellent strength, biocompatibility, biodurability, friction and wear characteristics for high performance, longer life and lower risk of adverse responses such as particulate induced inflammation and osteolysis. There is also need for such devices having articulating surfaces that do not produce potentially harmful metallic wear particulate. Ideally, known problems of using polymeric articulation surfaces, such as higher failure rates and the increased wear associated with strain hardening caused by multidirectional motion, could also be overcome. Such devices are needed for applications requiring conforming bearing surface, such as an acetabular cup for a hip joint, and also for high stress, non-conforming contact applications such as in a knee joint.
There is also an unmet need for devices that meet these requirements while also being substantially radiolucent for improved imaging of the affected area. Ideally, such devices would also have a modulus of elasticity closer to the adjacent bone tissue to minimize the adverse effects of stress shielding on the adjacent bone. There is a further need to reduce the number of components in such total joint arthroplasty devices so as to provide fewer modes of failure, to reduce parts inventory and simplify manufacturing and assembly. Such devices should also be readily sterilized using conventional radiation or steam sterilization techniques without causing oxidation and associated adverse effects. Ideally such devices could be provided for the major weight bearing joints in a range of sizes required to serve the full patient populations for various degenerative joint conditions.